This invention relates to a process and apparatus for producing low sulfur content hot reducing gas and especially that gas formed by the combustion and gasification of sulfur-bearing carbonaceous fuel. The gasification of solid carbonaceous fuel, such as by reaction with a limited quantity of oxygen to produce carbon monoxide, is well known. Either pure oxygen or air, with or without steam, may be utilized in the reaction. The products of combustion are reducing gases including carbon monoxide, hydrogen, carbon dioxide, water vapor and nitrogen. Hydrogen is produced from the hydrocarbons in the fuel, and also by reaction of injected steam with carbon, while nitrogen may be brought in by air and may also be contained in the fuel. Carbon dioxide and water vapor may also be used to react with the carbonaceous fuel and the reducing gas produced therefrom to vary the final composition of the product reducing gas.
One of the serious problems with gasification of carbonaceous fuels is that many commercially available carbonaceous fuels contain sulfur. Sulfur-containing reducing gases, usually predominantly hydrogen sulfide, are produced when these fuels are reacted with air or oxygen in a gasification process. These sulfur-containing gases in the reducing gas are objectionable for a number of reasons. One reason is that when the sulfur finally ends up in the atmosphere, it results in serious pollution problems. Additionally, this sulfur should be removed from the product gas before its use in many applications, such as metallurgical reducing gas where it will contaminate the metal produced, synthesis gas where it will poison the catalyst in the reaction system, or feed stock for pipeline gas where it may promote corrosion and other detrimental effects. Also, if the product gas is burned to raise steam or generate electricity, it is advantageous to remove the H.sub.2 S before combustion rather than having to remove SO.sub.2 from the larger volume of combusted gas. In at least two of these applications, i.e. as a reducing gas for direct reduction of iron ore or fuel for gas-turbine engines, it is desirable to remove the H.sub.2 S while the product gas is still hot so that gas can be used directly without loss of heat values.
To offset the cost of desulfurizing the hot reducing gases, the byproducts of sulfur removal should be marketable: (i) recovery of sulfur from the spent absorbent, (ii) regeneration of absorbent for recycle, or (iii) marketing the treated spent absorbent, after the recovery of sulfur, for other applications. The absorbent used for desulfurization of hot reducing gases should have the capability of lowering the sulfur content of the treated gas to below 100 ppm without much changing the reducing capacity or fuel value of the gas.
Attempts have been made to remove the sulfur during the gasification reaction itself. U.S. Pat. No. 3,533,730, incorporated herein by reference, is an example of such a process whereby the carbonaceous fuel is reacted with a controlled quantity of oxygen beneath the surface of a molten iron bath and whereby lime on the surface of the molten iron bath is used to desorb sulfur from the iron bath. Sulfur is then recovered from the coal ash-lime-sulfur molten slag byproduct. There are serious questions concerning the practical operability of this process. The rate of coal gasification depends upon the rate of coal dissolution for a given melt size, which are relatively slow compared with volumetric gasification rates for other processes. Furthermore, the sulfur in the slag byproduct is recovered only by costly additional steps. The gasification product generally contains fly ash which also requires an extra step for removal.
The use of calcined dolomite has been suggested for a regenerative cycle process of desulfurization of hot reducing gases. See U.S. Pat. Nos. 3,276,203; 3,296,775; 3,307,350; 3,402,998; and 3,853,538, each incorporated herein by reference. While dolomite is an effective gas-desulfurizing agent, the most commonly proposed method of regenerating dolomite, reacting with CO.sub.2 and H.sub.2 O under slightly reducing conditions at pressures greater than about 50 psig and temperatures preferably about 1000.degree.-1200.degree. F. to liberate H.sub.2 S, does not achieve complete regeneration of the dolomite. One of the problems is that calcium carbonate formed in the regeneration coats the regenerated dolomite thereby reducing its effectiveness. Furthermore, because the spent dolomite contains appreciable nonregenerated calcium sulfide, it must undergo expensive and complete treatment to bring it to a state suitable for disposal without causing pollution of the air and groundwater. When dolomite is calcined after having been regenerated by the above suggested process, some of the residual sulfur in the dolomite can be released, which requires difficult treatment to bring the stack gas to a condition suitable for venting to the atmosphere.
Copending and commonly assigned application Ser. No. 154,731, filed May 29, 1980, by E. T. Turkdogan and entitled "Low Sulfur Content Hot Reducing Gas Production Using Calcium Oxide Desulfurization", incorporated herein by reference, teaches a process for removing sulfur from a hot reducing gas stream by contacting the gas stream with a fixed bed of particulate calcium oxide desulfurizing agent, such as calcined dolomite. The desulfurizing agent is used one time and is then contacted with boiling water or wet steam, preferably under pressure, to remove the sulfur from the calcium sulfide composition produced in the gas desulfurizing step. A basic problem with this process is that when the reducing gas stream initially contains significant fly ash then the fixed bed rapidly becomes plugged with fly ash, thus resulting in plant shut downs and wasted desulfurizing agent. The invention described in copending and commonly assigned application Ser. No. 158,190, filed June 11, 1980, by J. Feinman and J. E. McGreal, Jr. and entitled, "Low Sulfur Content, Fly Ash Free Hot Reducing Gas Production Using Calcium Oxide Desulfurization", incorporated herein by reference, teaches a solution to the fly ash problem by using a moving bed of desulfurizing agent so that the fly ash is continually removed with the spent desulfurizing agent. After removal of the sulfur from the spent desulfurizing agent, the mixture of fly ash and desulfurizing agent is preferably disposed of and fresh desulfurizing agent is preferably used as input to the moving bed.
One of the basic problems that remains with the calcium oxide desulfurizing agent system is how to dispose of the sulfur containing water produced in the process of desulfurizing the calcium sulfide of the spent desulfurizing agent composition. Water desulfurizing methods are expensive. Adding such sulfur containing waste waters to streams or the like is generally unacceptable.
Another problem with the calcium oxide desulfurizing agent system is how to achieve sufficiently rapid desulfurization of the spent dolomite without excessive use of fresh water.